The scanning transmission electron microscope (STEM) has made significant contributions to structural biology by providing accurate determinations of the molecular masses of large protein assemblies that have arbitrary shapes and sizes. Nevertheless, STEM mass mapping has been implemented in very few laboratories, most of which have employed cold-field emission gun (FEG) electron sources operating at acceleration voltages of 100 kV and lower. Here we show that a 300 kV commercial transmission electron microscope (TEM) equipped with a thermally assisted Shottky FEG can also provide accurate STEM mass measurements. Using the elastic-scattering cross sections from the NIST database, we show that the measured absolute mass values for tobacco mosaic virus and limpet hemocyanin didecamers agree with the known values to within better than 10%. We find that the measured molecular weight of the hemocyanin assemblies agrees with the known value to within 3%. FEG TEMs operating at intermediate voltages (200-400 kV) are becoming common tools for determining the structure of frozen hydrated protein assemblies. The ability to perform mass determination with the same instrument can provide important complementary information about the numbers of subunits comprising the protein assemblies whose structures are being studied. [unreadable] [unreadable] We have used a combination of STEM mass mapping and energy-filtered transmission electron microscopy (EFTEM) to characterize specific generations (G5, G6, G7) of polyamidoamine (PAMAM) dendrimers, which are being developed for potential nanomedicine applications. PAMAM dendrimers can be chelated with diethylene triamine pentaacetic acid (DTPA) gadolinium (Gd) as a contrast agent for magnetic resonance imaging as well as with drugs for cancer therapy. STEM images provide the distribution of molecular weights of the diffferent dendrimer generations, whereas EFTEM imaging provides the numbers of bound Gd atoms. This approach provides a quantitative assessment of the degree of monodispersity of nanoparticles, which will be useful in developing these reagents for eventual applications in nanomedicine.